The present invention relates to a casting member used in contact with a molten metal, such as a mold used for die casting or casting, a core pin, a piston ring used in an injection machine, or the like.
Steels such as hot-die steel, high-speed steel, stainless steel and the like have been used in casting members used for molding by casting of a molten metal. A metal to be molded that is most frequently used for molding by casting is an aluminum alloy. In the case of the above-mentioned steel materials used in casting members such as molds, the steel material in a portion of the casting member in contact with the aluminum alloy is wasted by melting in a melt of the aluminum alloy to increase the iron content of the aluminum alloy melt and deteriorate the quality of a cast product. Moreover, the waste by melting of the molds and the like causes various operational troubles.
As a measure against the above problem, nitriding on the working plane of a casting member has been often conducted because it gives a deep hardened layer and can be conducted at a very low cost. However, since employment of a high-melting aluminum alloy as a material to be molded has become frequent for the purpose of increasing the strength of a cast product, nitriding has the following disadvantages: N in the hardened layer is easily diffused into an aluminum melt at a high temperature, so that the resistance to the waste by melting is deteriorated by the disappearance of the hardened layer, resulting in rapid progress of the waste-by-melting phenomenon.
In order to solve such a problem, employment of a mold having a working plane coated with a ceramic hardly reactive with a molten metal by physical vapor deposition (hereinafter referred to also as PVD method) has become frequent. There have been proposed, for example, a method in which a base material for mold is subjected to carburizing or nitriding and then a coating layer of TiC, TiN or the like is formed by PVD method (JP-A-61-033734), and a method in which the surface of a mold is coated with a Ti intermediate layer and then TiAlN (JP-A-07-112266). There have also been proposed methods in which CrN is used as a coating layer (JP-A-10-137915 and JP-A-2001-11599).
The use environment of a member used in contact with a melt, such as a casting mold has become very severe because of a recent increased speed of casting cycle and a recent reduced thickness of a mold due to near net shape. That is, the following troubles have come to be remarkably caused: the thermal expansion of a mold or other members by a molten metal during casting; and the deflection of the mold or the members by shrinkage thereof by application of a mold release agent after the solidification of a material to be processed or by the solidification and shrinkage of the material to be processed. As a result, in the coating layers having the compositions proposed in the prior art references 1 to 4, fine cracks are formed during use of a member having the coating layer, and a molten metal infiltrates into the member through cracks and reacts with iron in a base material just under the surface treatment layer of the member to form an alloy. When a portion just under the surface coating layer is expanded by the formation of the alloy, the following phenomenon is observed: the treatment layer is peeled, so that the waste of the member by melting proceeds rapidly.
In addition, the above-mentioned coating layers proposed have no sufficient resistance to seizure to cope with use environment that has become severe in recent years. Therefore, when an explanation is made by taking the case of a mold, seizure occurs between the working plane of the mold and a cast product at the time of release of the cast product from the mold in the early stages of use of the mold, resulting in mold correction cycles, a decrease of the life of the mold and the unsatisfactory shape of the cast product.
The present invention is intended to provide a casting member that is used with its working plane contacted with a high-temperature molten metal, such as a die-casting mold or a core pin, said casting member being free from the problems described above.